In the widely used commercial process for the manufacture of cement, the steps of drying, calcining, and clinkering cement raw materials are accomplished by passing finely divided raw materials, including calcareous minerals, silica and alumina, through a heated, inclined rotary vessel or kiln. In what is known as conventional long dry or wet process kilns the entire mineral heating process is conducted in a heated rotating kiln cylinder, commonly referred to as a “rotary vessel.” The rotary vessel is typically 10 to 15 feet in diameter and 200–700 feet in length and is inclined so that as the vessel is rotated, raw materials fed into the upper end of the kiln cylinder move under the influence of gravity toward the lower “fired” end where the final clinkering process takes place and where the product cement clinker is discharged for cooling and subsequent processing. Kiln gas temperatures in the fired clinkering zone of the kiln range from about 1300° C. (˜2400° F.) to about 2200° C. (˜4000° F.). Kiln gas exit temperatures are as low as about 250° C. (˜400° F.) to 350° C. (˜650° F.) at the upper mineral receiving end of so-called wet process kilns. Up to 1100° C. (˜2000° F.) kiln gas temperatures exist in the upper end of dry process rotary kilns.
Generally, skilled practitioners consider the cement making process within the rotary kiln to occur in several stages as the raw material flows from the cooler gas exit mineral feed end to the fired/clinker exit lower end of the rotary kiln vessel. As the mineral material moves down the length of the kiln it is subjected to increasing kiln gas temperatures. Thus in the upper portion of the kiln cylinder where the kiln gas temperatures are the lowest, the in-process mineral materials first undergo a drying/preheating process and thereafter move down the kiln cylinder until the temperature is raised to calcining temperature. The length of the kiln where the mineral is undergoing a calcining process (releasing carbon dioxide) is designated the calcining zone. The in-process mineral finally moves down the kiln into a zone where gas temperatures are the hottest, the clinkering zone at the fired lower end of the kiln cylinder. The kiln gas stream flows counter to the flow of in-process mineral materials from the clinkering zone, through the intermediate calcining zone and the mineral drying/preheating zone and out the upper gas exit end of the kiln into a kiln dust collection system. The flow of kiln gases through the kiln can be controlled to some extent by a draft induction fan positioned in the kiln gas exhaust stream. Over the last 10–20 years preheater/precalciner cement kilns have proven most significantly more energy efficient than the traditional long kilns. In precalciner kilns the raw mineral feed is heated to calcining temperatures in a stationary counterflow precalciner vessel before it drops into a heated rotary vessel for the higher temperature clinkering reactions.
Responsive to environmental concerns and more rigorous regulating of emission standards, the mineral processing industry has invested in a significant research and development effort to reduce emissions from cement and other mineral processing kilns. The present invention provides a method and apparatus for improving thermal efficiency and reducing emission of gaseous pollutants during the manufacture of thermally processed mineral products such as cement and limestone. The invention finds application to both so-called long mineral processing kilns and, in the case of cement manufacture, precalciner kilns, already recognized for their energy efficient production of cement clinker. The invention provides advantage in the form of reduced emissions and enhanced energy efficiency in supplemental fuels, the thermal processing of gas releasing minerals including, but not limited to, talconite, limestone, cement raw materials, and clays for the production of light weight aggregates.
In one aspect of the invention high energy/velocity air is injected into the kiln gas stream to reduce or eliminate stratification of gases in a kiln during thermal processing of a mineral that liberates a gas as it is processed.
In another aspect of this invention kiln gas mixing energy is delivered to the kiln gas stream by injecting air at high velocity into rotary kilns in a manner designed to impart rotational momentum to the kiln gases in the rotary vessel. It has been found that injection of high velocity air to promote cross-sectional mixing in mineral processing kilns works to improve energy efficiency by facilitating energy transfer to the mineral bed, and concomitantly such air injection alters the stoichiometry and temperature profile of combustion in the primary combustion zone to reduce the formation of byproduct nitrogen oxides.
According to one aspect of the present invention, there is provided a method for reducing NOx emissions and improving energy efficiency during mineral processing in a rotary kiln. The kiln comprises an inclined rotary vessel having a primary burner and a combustion air inlet at its lower end and an upper end for introducing raw mineral feed. The method finds particular use wherein the mineral in a mineral bed in the rotary vessel undergoes a gas releasing chemical reaction during thermal processing in the kiln. The method comprises the step of injecting air into the rotary vessel at a velocity of about 100 to about 1000 feet per second, typically from an air pressurizing source providing a static pressure of greater than about 0.15 atmospheres, and in one aspect of the invention, at a point along the lower one-half length of the rotary vessel, where the temperature difference between the kiln gases and the mineral are the greatest, to mix the gas released from the mineral with combustion gases from the primary burner. Preferably the mass flow rate of the injected air is about 1 to about 15% of the mass rate of use of combustion air by the kiln.
In one embodiment air is injected into the rotary vessel preferably through an air injection tube extending from a port in the rotary vessel wall into the rotary vessel and terminating in a nozzle for directing the injected air along a predetermined path in the rotary vessel. Typically air is injected into the rotary vessel through two or more nozzles positioned in the rotary vessel at a distance of about H to about 2H from the wall of the rotary vessel wherein “H” is the maximum depth of the mineral bed in the vessel. Preferably the predetermined path of the injected air is directed to impart rotational momentum to the combustion gases flowing through the rotary vessel. In one aspect of the invention the method further comprises the step of burning supplemental fuel delivered into the rotary vessel downstream relative to kiln gas flow in the kiln from where the air is injected into the kiln. In still another embodiment of the invention the method further includes the step of injecting air into the rotary vessel at a velocity of about 100 to about 1000 feet per second at a point downstream, relative to gas flow in the kiln, from the supplemental fuel delivery port to mix the gas released from both the mineral bed and the burning supplemental fuel with the combustion gases from the primary burner. The rate of injection of air into the kiln is generally about 1% to about 15%, more typically about 1% to about 7% of the mass of the total combustion air required per unit time during kiln operation. In one particular embodiment of the invention the air injection nozzles have an orifice with an aspect ratio greater than 1, for example, an orifice of rectangular or elliptical cross-section.
In another aspect of the invention there is provided a method for reducing NOx emissions and improving combustion efficacy in a preheater/precalciner (PH/PC) cement kiln. The precalciner kiln has a rotary vessel portion having a primary burner combustion zone and a stationary precalciner vessel portion having secondary burner combustion zone. Each of the primary burner and the precalciner portion is supplied with controlled amounts of preheated combustion air. In operation the combustion gases from the primary combustion zone flows serially through the rotary vessel, the precalciner vessel portion and into a series of cyclones in counter-flow communication with a mineral feed. The method of the present invention as applied to a precalciner kiln comprises the step of injecting compressed air into the precalciner vessel portion of the kiln at a point before the first cyclone, at a mass rate corresponding to about 1% to about 7% of the total combustion air per unit time required by the kiln. Preferably the air is injected at a velocity of about 100 to about 1000 feet per second through two or more air injection nozzles. In one embodiment the air is compressed to a pressure of about 4 to about 150, more typically about 40 to about 100 pounds per square inch before being injected into the precalciner vessel portion. Preferably the nozzles are directed into the precalciner vessel to optimize cross-sectional mixing of the contained gases and fluidized mineral. In one embodiment the nozzles are positioned to promote turbulent flow in the vessel and in another embodiment the nozzles are directed into the precalciner vessel to promote rotational or cyclonic flow in said vessel.
In an alternate embodiment of the present invention there is provided a modified precalciner cement kiln wherein the modifications comprise an air injection nozzle positioned in or on the stationary precalciner vessel and means for delivering compressed air to the nozzle and into the vessel at a linear velocity of about 100 to about 1000 feet per second. Preferably the modified kiln is fitted with a plurality of nozzles positioned to deliver compressed air into the precalciner vessel.
In still another embodiment of the present invention there is provided a mineral processing kiln modified for operation with reduced NOx emissions and increased energy efficiency. The kiln comprises an inclined rotary vessel having a primary burner and combustion air inlet at its lower end. The kiln finds particular application to the thermal processing of minerals that undergo a gas releasing chemical reaction during thermal processing. The kiln is modified to include an air injection tube for injecting air into the rotary vessel at a velocity of about 100 to about 1000 feet per second. The injection tube extends from a port in the wall of the vessel and into the rotary vessel terminating in a nozzle for directing the injected air along a predetermined path in the vessel. The port is preferably located at a point along the lower one-half length of the rotary vessel to mix gas released from the mineral bed with combustion gases from the primary burner. Additional modifications of the kiln include a fan or compressor in air flow communication with the air injection tube and a controller for the fan or compressor to adjust the rate of air injection into the kiln. The fan or compressor can be stationary and in air flow communication with the port in the wall of the vessel via, for example, an annular plenum aligned with the path of the port during rotation of the vessel. Alternatively, the fan or compressor can be mounted on the wall of the rotary vessel for direct air injection into the kiln. Power is delivered to fan or compressor mounted on the surface of the vessel via a circumferential power ring.
Preferably the modified mineral processing kiln is modified to include two or more air injection tubes for injecting air into the rotary vessel, each injection tube terminating in a nozzle for directing the injected air along a predetermined path in the vessel. Preferably the nozzle or nozzles are positioned in the rotary vessel at a distance of about H to about 2H from the wall of the rotary vessel wherein “H” is the maximum depth of the mineral bed in the rotary kiln vessel. The air injection nozzles are preferably positioned so that the predetermined path of the injected air from each nozzle works to impart rotational momentum to the combustion gases flowing through the rotary vessel.
The air injection tubes can be mounted to extend from the port into the rotary vessel perpendicular to a tangent to the rotary vessel at the port and terminate in a nozzle for directing the injected air along a predetermined path in the vessel selected to impart rotational momentum to the kiln gas stream. Alternatively, the injection tube(s) can be positioned to extend from the port in the rotary vessel into the vessel at an acute angle to a tangent at the port and substantially perpendicular to a radius line of the rotary vessel extending through the end of the tube. Air injection tubes so configured work to direct the injected air across the kiln gas stream to impart rotational momentum to the kiln gas stream at the point of injection. In one embodiment, the orifice of the injection tube is formed to have an aspect ratio greater than one.
The injection tube is formed to communicate with a source of pressurized air, preferably a fan, blower, or compressor capable of providing a static pressure differential of greater than about 0.15 atmospheres, preferably greater than about 0.20 atmospheres. The fan, blower, or compressor is sized and powered sufficiently to deliver injected air continuously into the kiln with a kinetic energy input of about 1 to about 10 watt/hour per pound of injected air (corresponding to about 0.1 to about 1 watt/hour per pound of kiln gas.) The size of the orifice of the air injection nozzles are selected so that the mass flow rate of injected air at the applied static pressure is about 1 to about 15%, more preferably about 1 to about 10% into the rotary vessel or about 1 to about 7% where air is injected into the stationary preheater/precalciner portion. The linear velocity of the injected air typically ranges from about 100 feet per second to about 1000 feet per second.
In one embodiment the modified mineral processing kiln further comprises a supplemental fuel delivery port and a tube extending from the port into the rotary vessel at a point on the vessel downstream, relative to gas flow in the kiln, from the location of the air injection tube. The kiln can be further modified to include one or more additional air injection tubes for injecting air into the rotary vessel at high velocity under the influence of a fan or compressor in gas flow communication with the air injection tube. The injection tube terminates in a nozzle for directing the injected air along a predetermined path in the vessel. The air injection tube is located at a point on the rotary vessel downstream, relative to gas flow into the kiln, from the supplemental fuel delivery port to mix gases released from both the mineral bed and the burning supplemental fuel with the combustion gases from the primary burner. A controller is provided for the fan or compressor to adjust the rate of air injection into the kiln at the downstream air injection point.
In one other aspect of the invention there is provided a method for reducing NOx in the effluent gas stream from a long rotary cement kiln modified for burning supplemental fuel. The kiln in operation comprises an inclined cylindrical vessel rotating about its long axis. The vessel is heated at its lower end by primary burner and charged with raw material at its upper end. A kiln gas stream flows from the heated lower end having a primary burner and a combustion air inlet through the upper end of the vessel. The in-process mineral material forms a mineral bed flowing at a maximum depth H under the influence of gravity in the vessel counter-current to the kiln gas stream from a drying zone in the upper most portion of the rotary vessel. The mineral bed flows through an intermediate calcining zone, and into a high temperature clinkering zone before exiting the lower end as cement clinker. Supplemental fuel is charged into the vessel through a port and a drop tube in communication with the port in the vessel wall to burn in contact with calcining mineral in a secondary burning zone coincident with at least a portion of the calcining zone. Application of the present invention to reduce NOx in the effluent gas stream from the kiln comprises the step of injecting air at a velocity of about 100 to about 1000 feet per second through an air injection tube extending from a port in the vessel and terminating in a nozzle for directing the injected air along a predetermined path in the vessel. The air injection port is located at a point downstream relative to kiln gas flow of the clinkering zone and upstream relative to kiln gas flow of the upper end of the calcining zone. The air injection nozzle is positioned in the vessel a distance from about H to about 2H from the wall of the vessel and the predetermined path of the injected air preferably forms an angle of greater than 45 degrees with a line segment parallel to the rotational axis of the vessel and extending from the point of injection through the mineral feed in the vessel. The rate of injection of the air into the vessel is controlled to be about 1% to about 10% of the mass of the total combustion air used per unit time during kiln operation.
In another aspect of the disclosure, there is provided a method of imparting momentum to kiln gases which are exiting the rotary vessel of the kiln. The kiln in operation includes a stationary portion positioned proximate the upper end of the rotary vessel. The stationary portion includes a stationary vessel and includes injectors in communication with the stationary vessel. The injectors are configured to introduce a quantity of unheated air into the stationary vessel. The quantity of unheated air imparts momentum to kiln gases which are exiting the rotary vessel and flowing through the stationary vessel.